1. Field of the Invention
The invention relates to an electronic ballast, and more particularly to an electronic ballast for a high intensity discharge (HID) lamp.
2. Description of the Related Art
High intensity discharge (HID) lamps have been widely used to replace incandescent lamps because HID lamps have high light-emitting efficiency, high color rendering index, and a long service life. However, the acoustic resonance phenomenon occurring in HID lamps may cause various problems, such as arc instability, color temperature variations, light output fluctuation, arc extinction, or in the worst case, cracked arc tubes.
To avoid the acoustic resonance, a low frequency square wave voltage having a frequency lower than 1 KHz is used to drive an HID lamp. FIG. 1 illustrates a conventional electronic ballast capable of generating a low frequency AC voltage for driving an HID lamp 200′. The conventional electronic ballast includes a filtering circuit 11, a bridge rectifier circuit 12, a boost converting circuit 13, a buck converting circuit 14, and a full-bridge converting circuit 15.
The filtering circuit 11 filters an AC input voltage from a voltage source 100′ to eliminate high frequency electromagnetic interference.
The bridge rectifier circuit 12 is coupled to the filtering circuit 11. The bridge rectifier circuit 12 receives and rectifies the AC input voltage filtered by the filtering circuit 11, and outputs a rectified DC voltage.
The boost DC-to-DC converting circuit 13 is coupled to the bridge rectifier circuit 12 to receive the rectified DC voltage from the bridge rectifier circuit 12. The boost DC-to-DC converting circuit 13 has power factor correction function for boosting the rectified DC voltage to output a boosted DC voltage.
The buck DC-to-DC converting circuit 14 is coupled to the boost DC-to-DC converting circuit 13 to receive the boosted DC voltage from the boost DC-to-DC converting circuit 13. The buck DC-to-DC converting circuit 14 reduces the boosted DC voltage to output a buck DC voltage.
The full-bridge converting circuit 15 is coupled between the buck DC-to-DC converting circuit 14 and the HID lamp 200′. The full-bridge converting circuit 15 receives the buck DC voltage from the buck converting circuit 14, and converts the buck DC voltage into a low frequency AC voltage in the form of an AC square wave signal. The low frequency AC voltage is output to the HID lamp as a driving voltage.
In such a configuration, the conventional electronic ballast has a relatively complicated circuit construction that consists of a large number of components, thereby resulting in a relatively high cost. In addition, the conventional electronic ballast generates the driving voltage for the HID lamp 200′ through multi-stage conversion, thereby resulting in reduced power conversion efficiency.